Drug delivery has been a persistent challenge in the pharmaceutical arts, particularly when a drug is unstable and/or poorly absorbed at the locus in the body to which it is administered. One such class of drugs includes 2,5-diketopiperazines having the general structure of Formula 1.

These 2,5-diketopiperazines have been shown to be useful in drug delivery, particularly those bearing acidic R groups (see for example U.S. Pat. Nos. 5,352,461 entitled “Self-assembling diketopiperazine drug delivery system;” 5,503,852 entitled “Method for making self-assembling diketopiperazine drug delivery system;” 6,071,497 entitled “Microparticles for lung delivery comprising diketopiperazine;” and 6,331,318 entitled “Carbon-substituted diketopiperazine delivery system,” each of which is incorporated herein by reference in its entirety for all that it teaches regarding diketopiperazines and diketopiperazine-mediated drug delivery). Diketopiperazines can be formed into particles that incorporate a drug or particles to which a drug can be adsorbed. The combination of a drug and a diketopiperazine can impart improved drug stability. The particles can be administered by various routes of administration. As dry powders these particles can be delivered to specific areas of the respiratory system, depending on particle size. The particles can be made small enough for incorporation into intravenous suspension dosage forms. Oral delivery is also possible using a suspension, or as particles pressed into tablets or contained in a capsule. These diketopiperazines can also facilitate absorption of the associated drug.
A conventional step in synthesis of 2,5-diketopiperazines includes the preparation of N-protected bis-3,6-[4-aminobutyl]-2,5-diketopiperazine by thermal condensation of lysine. Conventional commercial manufacturing processes utilize (N-benzyloxycarbonyl)-lysine (Cbz-L-lysine) to produce bis-3,6-[(N-benzyloxycarbonyl)-4-aminobutyl]-2,5-diketopiperazine (DKP1). This process entails heating Cbz-L-lysine in m-cresol for 18-22 hours at 160° C.-170° C., and provides DKP1 in an average yield of about 47.5% after recrystallization from glacial acetic acid.
Therefore, it is an object of the present invention to provide improved methods of synthesis for diketopiperazines.